I actually didn't have much trouble against That Man, or Diva.
The trick with That Man is to not be locked on all the time since Sol walks during lock on and runs otherwise, making it much easier to dodge the firepower (especially the moving walls) and take out those assistants.
Diva was really cool during the ground running, but like it's already been said once you figure out her patterns (which are telegraphed a mile away to boot), she's really easy.
This is all with my experience in Hard Mode as I skipped easy and normal.
However, the mission that was the bane of my existence was mission 8 "Water Drop".
Holy crap I hated that mission.
The prior mission Dr. Paradigm is quite effective as his troops just swarmed the balls off you, yet as soon as they make him your partner he can't even defend himself against a tiny group of reinforcements?
So you gotta constantly give him your personal attention with Sol all while keeping your own base from being crushed.
Gah.